1. Field of the Invention
The present invention relates to a process for continuously producing geometric shaped catalyst bodies K,
which comprise, as an active material, a multielement oxide which comprises, as elements E other than oxygen, the element Mo, at least one of the two elements Bi and V, and at least one further element from the group consisting of Co, Ni, Fe, Cu and the alkali metals, in process stages A) to E), in which                in process stage A), with the aid of sources Q of the elements E, a finely divided mixture M is obtained with the proviso that at most 10% by weight of the total weight of the particles present in the finely divided mixture M have a particle diameter of dM≧160 μm and the particle diameter d50M of the particles of the mixture M satisfies the condition 1 μm≦d50M≦150 μm;        in process stage B), the finely divided mixture M*, which consists either only of the finely divided mixture M or of a mixture of the finely divided mixture M and fines F which are obtained in the next process stage C) and are recycled into process stage B) continuously or batchwise from process stage C), is compacted by press agglomeration in which the maximum pressure applied is P1 to agglomerates A whose longest dimension L is ≧3 mm;        in process stage C), the agglomerates A are comminuted and the particulate material formed in the comminution is separated by sieving into a powder P whose particle diameters dP are ≦2 mm and, to an extent of at least 90% by weight, based on the weight of the powder P, ≧160 μm, as sieve oversize, and into fines F as sieve undersize, and the fines F are recycled continuously or batchwise into process stage B to obtain finely divided mixture M*;        in process stage D), the powder P conducted into it or a mixture P* consisting of the powder P conducted into process stage D) and shaping assistants is used to obtain, by press agglomeration in which the maximum pressure applied is P2 and satisfies the relationship P2≧2·P1, geometric shaped bodies V with the proviso that        when the powder P is conveyed into process stage D) and when shaping assistants are mixed into the powder P, a particle diameter dP≧160 μm is maintained overall in at least 40% by weight (preferably at least 60% by weight, more preferably at least 80% by weight, or at least 90% by weight or 100% by weight) of the particles of the powder P, based on the weight thereof; and        in process stage E), at least a portion of the shaped bodies V is treated thermally at elevated temperature to obtain the geometric shaped catalyst bodies K.        
2. Description of the Background
Processes for producing geometric shaped catalyst bodies which comprise, as an active material, a multielement oxide which comprises, as elements other than oxygen, the element Mo, at least one of the two elements Bi and V, and at least one further element from the group consisting of Co, Ni, Fe, Cu and the alkali metals are known (cf., for example, EP-A 184 790, US-A 2005/0263926 and JP-A 10/29097):
In general, this involves, with the aid of sources of the elements other than oxygen in the multielement oxide (sources=starting compounds which comprise at least one of the elements and which are either already oxides or those compounds which are converted to oxides by thermal treatment at elevated temperature, at least in the presence of molecular oxygen), producing a finely divided intimate mixture which comprises the elements other than oxygen in the multielement oxide in the required stoichiometry. Press agglomeration then forms, from the finely divided intimate mixture, geometric shaped bodies (shaped catalyst precursor bodies) of the desired geometry. Thermal treatment of the resulting shaped catalyst precursor bodies then affords the desired geometric shaped catalyst bodies therefrom.
Such geometric shaped catalyst bodies find use, for example, for charging (if appropriate diluted with inert shaped bodies) of the interior of the reaction tubes of a tube bundle reactor with a fixed catalyst bed. Such a fixed catalyst bed is suitable, inter alia, for performing heterogeneously catalyzed gas phase reactions (e.g. partial oxidations of organic compounds). Instead of a tube bundle reactor, it is also possible to charge a thermoplate reactor.
The appropriate reaction gas mixture flows through the fixed catalyst bed and the desired reaction proceeds during the residence time over the catalyst surface.
A disadvantage of shaped bodies obtained by mechanical compaction of a pulverulent aggregate is quite generally that the cohesion of the powder grains in the resulting shaped body is accomplished essentially not by intramolecular chemical conditions, but by remaining interparticulate bonds. Although particle deformations and fracturing operations in the compacting operation generally result in an increase in the interparticulate overall contact area, the magnitude of the interparticulate binding forces generated by the compaction is comparatively limited.
As a consequence, shaped catalyst precursor bodies obtained as described in some cases have damage, for example cracks, which are frequently barely perceptible visually. In a subsequent thermal treatment of such shaped precursor bodies, in the course of which gaseous compounds attributable to constituents which decompose and/or are converted chemically in the course of the thermal treatment are generally also released in the shaped catalyst precursor body, damage already present, for example crack formation, generally increases and in many cases develops to become a fracture. Catalyst fragments present in a fixed catalyst bed, however, result in compaction thereof and ultimately cause an increase in the pressure drop experienced in the reaction gas mixture as it flows through said bed.
A countermeasure which can be taken to reduce the above-described phenomenon consists, for example, in, prior to the introduction of the geometric shaped catalyst bodies K obtained into the fixed catalyst bed, sieving off the fragments formed in the course of production thereof (cf., for example, U.S. Pat. No. 7,147,011 and DE-A 10 2007 028 332). However, a disadvantage of such a procedure is that the raw material costs for an industrial scale production of shaped catalyst bodies are not inconsiderable, and therefore catalyst fragments obtained as sieve undersize (which passes through the sieve) in the course of sieving means a not inconsiderable material loss.
Furthermore, the measure of sieve removal of catalyst fragments cannot be employed when the thermal treatment of the shaped catalyst precursor bodies is undertaken actually within the reactor (for example in the reaction tube) (for example by passing appropriately heated gases through the reaction tubes charged with shaped precursor bodies).
In addition to the possible measure of sieve removal of catalyst fragments formed, another remedy which exists in principle for the above-described pressure drop problems is the possibility of taking measures which reduce the occurrence of catalyst fragments. Such measures recommended in the prior art are, for example, the additional use of shaping assistants, for example graphite, and the use of skillfully configured dies in the shaping (cf., for example, DE-A 10 2008 040 093 and DE-A 10 2008 040 094).
However, a disadvantage of these auxiliary measures is that they are incapable of remedying the problem described in an entirely satisfactory manner (the occurrence of catalyst fragments is not completely suppressed by the measures described and, moreover, it requires the use of specific shaping dies).
It was therefore an object of the present invention to provide an improved process for continuously preparing geometric shaped catalyst bodies K, which still has the disadvantages described to a reduced degree at worst.
In-depth studies have led to the result that the desired improvement can be achieved by, after process stage D) and prior to process stage E), separating the shaped bodies V obtained in process stage D) in a separation stage as process stage F) into non-intact shaped bodies V− and into intact shaped bodies V+, and supplying essentially only the latter to process stage E). One advantage of the separation measures mentioned is that the proportion of catalyst fragments ultimately obtained can be reduced by them. More particularly, however, it is advantageous in that, in contrast to catalyst fragments, shaped bodies V− removed as described can be recycled into the process for producing geometric shaped catalyst bodies K (without significantly reducing the performance of the resulting geometric shaped catalyst bodies K), and thus mean no material loss. Specifically, when the non-intact shaped bodies V− are comminuted in a process stage G) to form a finely divided aggregate H whose particle diameter d50H satisfies the condition 1 μm≦d50H≦150 μm and which comprises particles with a particle diameter dH of ≧160 μm to an extent of at most 10% by weight of its total weight, and the finely divided aggregate H is recycled continuously or batchwise to the additional incorporation into the finely divided mixture M* to be subjected to the press agglomeration while ensuring that the total content of the finely divided aggregate H in the finely divided mixture M* does not exceed a maximum value of 20% by weight.
The above content restriction is of relevance especially because the material present in the aggregate H recycled as described, in the course of the overall process, undergoes multiple compaction which does not impair the performance of the resulting shaped catalyst bodies K according to in-house studies.
While it is advantageous for the catalytic properties of the multielement oxide active material present in the shaped catalyst bodies K when the shaped bodies V are produced starting from a very finely divided mixture of comparatively homogeneous character, it is more favorable for the flow properties of the mixture to be compacted when it also comprises relatively coarse components (cf. WO 2008/014839). Appropriately in application terms, the starting materials in the production of geometric shaped catalyst bodies K are therefore comparatively finely divided starting mixtures which are subsequently coarsened by a first press agglomeration with downstream comminution at first only in order to improve their flow properties. The latter ensures, for example, reproducible filling of the die cavity (“the powder fills it like a liquid”) in which the compaction to the shaped body V is then effected. Since the maximum pressure applied in the course of compaction to the shaped body V is significantly greater than that applied to coarsen the powder, no restriction in the quantitative proportion is required in the finely divided mixture M* with regard to material recycling from the powder coarsening.